Diversity of La Niña onset

Abstract

Three La Niña onset types were identified by the K-means cluster analysis of equatorial sea surface temperature anomaly evolutions during the past 111 years (1910–2020). The first type is characterized by a slow basin-wide transition from a neutral year to La Niña, driven by tropical North Atlantic (TNA) warming, which induced anomalous easterlies in the equatorial western Pacific through atmospheric Kelvin wave responses. The easterly anomalies initiate a cooling by triggering upwelling oceanic Kelvin waves, shoaling the thermocline, and strengthening the westward zonal currents. The second and third types are a transition from a central Pacific (CP) and a super eastern Pacific (EP) El Niño to La Niña. The former is attributed to the CP El \({\rm{Ni}}\tilde{{\rm{n}}}{\rm{o}}\) induced anomalous easterlies in EP that strengthened local surface latent heat flux and anomalous upwelling, whereas the latter is attributed to a substantially shoaling of ocean thermocline associated with the discharge of the preceding super El \({\rm{Ni}}\tilde{{\rm{n}}}{\rm{o}}\). These characteristics differentiate diversified types of La Niña onset.